Directions

Preheat oven to 400 degrees. Coat a 10-by-15-inch rimmed baking sheet with cooking spray, and line with parchment. Spray and flour parchment; shake off excess flour.

Heat milk in a saucepan until hot but not boiling; set aside. Place flour in an airtight container with the water; shake until dissolved. Set aside. Heat 2 tablespoons butter in a medium saucepan over medium heat. Add flour mixture; whisking constantly, cook until roux is lightly colored, 1 to 2 minutes. Whisk in hot milk; cook, stirring constantly, until thick, about 3 minutes more. Remove from heat; season with 1 teaspoon salt and cayenne. Lightly beat yolks in a bowl; gradually whisk some of the hot mixture into yolks to warm them. Whisk yolk mixture back into milk; set pan aside.

In a medium bowl, whisk egg whites until foamy. Add cream of tartar; whisk until smooth, firm peaks form. Stir a quarter of the whites and half the Parmesan into milk mixture; gently fold in remaining whites. Pour mixture onto prepared baking sheet; spread to fill all corners. Sprinkle with remaining Parmesan. Bake until top is browned and puffed, about 15 minutes. Remove from oven; let cool. Carefully invert onto a parchment-lined work surface; gently remove paper from bottom; set aside.

Bring a large saucepan of water to a boil. Fill a large bowl with ice and water; set aside. Add spinach to boiling water in bunches; cook until bright green and wilted, about 1 minute. Transfer to ice bath to stop cooking. Transfer to colander to drain; squeeze out excess water. Very finely chop spinach; set aside in a bowl.

Coat a baking sheet that is at least 15 inches long with cooking spray; set aside. Heat remaining tablespoon butter in a nonstick skillet over medium heat. Add onion, and cook, stirring occasionally, until tender, about 10 minutes. Remove from heat, and transfer to bowl with spinach. Add ricotta, and season with remaining 1/2 teaspoon salt and black pepper. Stir until well combined. Spread the filling over cooled souffle base. Using the parchment as a guide, roll souffle tightly, starting with a long side, and transfer carefully to baking sheet, seam side down. Brush with cream, and bake until filling is heated through and souffle is light golden, 25 to 30 minutes. Slice, and serve hot or at room temperature with sauce, if using.